1. Field of the Invention
Embodiments of the invention are generally in the field of Optical Coherence Tomography (OCT) and, more particularly pertain to Frequency Domain-Optical Coherence Tomography (FD-OCT) apparatus, methods, and applications thereof and, even more particularly to Frequency Domain-Doppler Optical Coherence Tomography (FD-DOCT) and Polarization-Sensitive Optical Coherence Tomography (PS-OCT) apparatus, methods, and applications.
2. Technical Background
Optical coherence tomography (OCT) is an imaging modality that can provide in vivo, noninvasive, high-resolution cross-sectional images of biological tissues. OCT imaging contrast relies on the variation of the strength of back-reflected light from a sample arising from refractive index fluctuation inside a biological sample. OCT can be used for structural mapping and, in a Doppler mode, to measure flow location, velocity, direction and profile.
Structural Mapping in the Frequency Domain
For structural mapping applications, while imaging can be done in both time domain and frequency domain, frequency domain OCT (FD-OCT) can achieve higher sensitivities. One of the main problems in FD-OCT, however, is the obscuring object structure caused by the mirror image generated by the Fourier transform in the image reconstruction process. A similar problem of the removal of twin images in holography has been investigated using hardware methods as well as numerical methods.
The removal of the minor image in FD-OCT is necessary to secure sufficient imaging depth especially in high axial resolution FD-OCT with a broadband source where it is difficult to achieve a fully resolved spectrum at the deeper points in space. Removal of the mirror image has been recognized as necessary to increase the imaging range required for long depth-of-focus optics in the sample arm of an FD-OCT system.
FD-OCT systems using phase shifting or frequency shifting methods have been developed to remove the minor image. Phase shifting has different implementations; it may be accomplished either by displacing the reference minor using a piezo translator or by exploiting the inherent phase shifts of 3×3 fiber-optic couplers. Both implementations directly or indirectly derive the real and imaginary components of the complex signal, which are always π/2 out of phase with each other. The phase shifting implementation using a piezo translator in the reference arm, however, requires two sequential measurements for a single full range FD-OCT image, which decreases imaging speed and is sensitive to any interferometer drifts between the π/2 phase shifted acquisitions. The implementation using the inherent phase shifts of 3×3 fiber-optic couplers enabled the instantaneous retrieval of the complex interferometric signal. The two signals were simultaneously obtained respectively in the two detectors. However the uneven wavelength-dependent splitting ratios in the 3×3 fiber coupler lead to imperfect performance. Also it can be employed only in fiber based OCT.
The frequency shifting method of removing the minor image uses an acousto-optic or an electrooptic phase modulator in the swept source based FD-OCT or a sequential modulation of the phase offset of the reference beam (M-scan) during lateral scanning of the probing beam (B-scan), which is referred to as the B-M mode scanning method. Although the two frequency shifting implementations require only one measurement to make a full range FD-OCT image, any phase error in the sequential phase modulation in the B-M method, which can be generated by movements of the subject, may limit the performance of the full range OCT. The implementation using an acousto- or electro-optic modulator in the reference arm can be applied only to the swept source based FD-OCT.
Flow-Mapping Doppler OCT in the Spectral Domain
Among the many functional OCT systems, Doppler OCT (DOCT) is one of the most useful. It is capable of the in-vivo monitoring of flow activity in biological samples such as blood flow in the human retina and the cardiovascular system of animal embryos. DOCT provides information about flow location, velocity, direction, and profile that cannot be obtained by intensity mapping alone.
Recent development in DOCT is mostly based on phase sensitive detection. This technique relies on the accuracy and stability of the measured phase difference between points at the same depth and lateral position in two consecutive axial scans. Without phase unwrapping, the maximum detectable velocity is governed by the time interval between the two scans. Recently, a time domain DOCT (TD-DOCT) with a speed of 30 frames per second, a velocity sensitivity of 17 μm/s, and a non-aliasing range of 3.9 mm/s was reported.
The Doppler technique was also extended to spectral domain OCT (SD-OCT), which not only has speed and sensitivity advantages over TD-OCT but also allows direct access to the phase information immediately following the Fourier transform. The maximum detectable velocity was also improved through the shortening of the acquisition time between two consecutive axial scans. A spectral domain Doppler OCT (SD-DOCT) utilizing a continuous readout CCD camera and achieving an acquisition speed of 29.3 kHz line rate has been reported. Further Improvement of imaging speed as well as the maximum detectable velocity in SD-DOCT by using a high speed CMOS camera as a detector resulted in a reported acquisition speed as high as 200 kHz, which was capable of 4D imaging of retina blood flow at about 13 volumes per second. A key challenge in conventional SD-OCT is the obscured object structure known as a minor image or ghost image arising from the Fourier transformation of the real function. Removal of the minor image in SD-OCT is desirable to achieve sufficient imaging depth particularly when employing a broadband light source where the achievement of high spectral resolution of a spectral interference signal is challenging. It is commonly known that the flow sensitivity of DOCT relies on the signal-to-noise ratio within the flow region. Removal of the minor image enables the use of the region around the zero path delay, which is the most sensitive region and, which, cannot be obtained in conventional SD-OCT, for flow imaging. Furthermore, the existence of a mirror image may obscure flow visibility in a sample such as capillary vessels where the flow diameter is relatively small, and the vessels may be completely overlapped by a stationary part of the sample.
Recently, a spectrometer-based full-range DOCT using the BM-scan method was demonstrated for imaging of the deep posterior of a human eye. The technique involves a filter process that causes a reduction in the detectable range of Doppler phase shift as compared to conventional SD-OCT. A reported different approach to full-range DOCT was based on a time-frequency analysis DOCT built on a spectrometer-based SD-OCT system. Contrary to phase sensitive detection, the Doppler phase shift information was determined from the amplitudes of Fourier transformations. The disadvantage to this method is that the full range signal is achieved by moving the reference mirror at a constant speed that causes the reduction in the detectable velocity dynamic range of the Doppler signal by half when operating in the full-range mode.
In view of the foregoing, the inventors have recognized the benefits and advantages that would be afforded by a Frequency Domain-Optical Coherency Tomography (FD-OCT) apparatus and methods that eliminate the minor image problem, provide high quality, full range OCT and DOCT images, and are capable of applications in, e.g., endoscopy, that can provide higher resolution than competing techniques and equipment, e.g., intravascular ultrasound.